Quantum Well Infrared Photodetector Focal Plane Arrays (QWIP FPA's) are conventionally used for infrared detection and imaging. Typical applications of QWIP FPA's include fiber optics communications systems, temperature sensing, night vision, eye-safe range finding, and process control. As is known in the art, QWIP FPA's are composed of arrays of detector structures, wherein each detector structure produces a signal that is transmitted through a conductor bump to an external Read Out Integrated Circuit (ROIC) unit cell. The outputs of the plurality of ROIC unit cells associated with each detector in the array produce an integrated representation of the signal from the detector. To produce this output signal, a fixed bias is applied to the detector and the detector photocurrent resulting from the bias and the incident radiation is integrated. This integration function is performed by an integration charge well (integration capacitor) that is disposed within each individual ROIC unit cell. The combined integrated outputs of the plurality of ROIC unit cells in the array produce an image corresponding to the received infrared radiation.
As shown in FIG. 1, a conventional photodetector architecture consists of the detector structure 1 physically separated from the ROIC unit cell structure 2 and electrically connected through a conductive bump 3. In this prior art photodetector architecture the integrating capacitor 4 (unit cell charge well) is physically disposed within the ROIC unit cell itself. The usable area of the ROIC unit cell is constrained by the pitch of the overlying detector in the array. This constraint on the usable area further limits the size of the charge well which is the largest component in the unit cell. As the pitch of each detector in the array is reduced to create greater detector density, the usable area of the associated unit cell must also be reduced. This reduction limits the size of the charge well and, ultimately, places limits on the density of the detector array.
To produce very high density (for example 856.times.480 or 1024.times.576) QWIP Focal Plane Arrays the detector pitch may need to be reduced to less than 18 .mu.m. As already noted, this small pitch will limit the usable area for the unit cell charge well and also any additional unit cell storage well capacitors. Even with the use of 0.5 .mu.m or 0.35 .mu.m technology the available area for these capacitors in the unit cell will be very small. Additionally, there is a need to place smart focal plane array functions into the ROIC but in conventional designs, the small pitch limits the space that is available to provide these functions. Thus, the conventional photodetector architecture, in which the unit cell contains all of the components except the detector, imposes a limitation on the size and functionality of the unit cell charge well and the density of the FPA.
An additional drawback of the prior art concerns the inherent variation in the conductivities of each of the detectors in a focal plane array. These variations in conductivity result in detector elements that have different responsivities to incident radiation (e.g., high responsivity/"hot" or low responsivity/"cold" detector pixels). Variation in responsivity among the detectors across the array disadvantageously leads to nonuniform array imagery. However, the ROIC circuitry of the prior art fails to provide any compensation for this variation in responsivity. The conventional photodetector of FIG. 1 includes a ROIC injection transistor 26 that is used to bias the detector element 1. This transistor functions to provide a constant bias voltage that produces a linear response from the detector element 1. Since the detector's responsivity is also a function of the applied bias voltage, the fixed bias provided by the prior art injection transistor 26 does not compensate for the variation in responsivity due to inherent variations in detector conductivity.